1. Field of the Invention
The present invention relates to a method and device for enhancing periodicity of the excitation of a signal synthesis filter in view of producing a synthesized wideband signal.
2. Brief Description of the Prior Art
The demand for efficient digital wideband speech/audio encoding techniques with a good subjective quality/bit rate trade-off is increasing for numerous applications such as audio/video teleconferencing, multimedia, and wireless applications, as well as Internet and packet network applications. Until recently, telephone bandwidths filtered in the range 200-3400 Hz were mainly used in speech coding applications. However, there is an increasing demand for wideband speech applications in order to increase the intelligibility and naturalness of the speech signals. A bandwidth in the range 50-7000 Hz was found sufficient for delivering a face-to-face speech quality. For audio signals, this range gives an acceptable audio quality, but still lower than the CD quality which operates on the range 20-20000 Hz.
A speech encoder converts a speech signal into a digital bitstream which is transmitted over a communication channel (or stored in a storage medium). The speech signal is digitized (sampled and quantized with usually 16-bits per sample) and the speech encoder has the role of representing these digital samples with a smaller number of bits while maintaining a good subjective speech quality. The speech decoder or synthesizer operates on the transmitted or stored bit stream and converts it back to a sound signal.
One of the best prior art techniques capable of achieving a good quality/bit rate trade-off is the so-called Code Excited Linear Prediction (CELP) technique. According to this technique, the sampled speech signal is processed in successive blocks of L samples usually called frames where L is some predetermined number (corresponding to 10-30 ms of speech). In CELP, a linear prediction (LP) synthesis filter is computed and transmitted every frame. The L-sample frame is then divided into smaller blocks called subframes of size N samples, where L=kN and k is the number of subframes in a frame (N usually corresponds to 4-10 ms of speech). An excitation signal is determined in each subframe, which usually consists of two components: one from the past excitation (also called pitch contribution or adaptive codebook or pitch codebook) and the other from an innovative codebook (also called fixed codebook). This excitation signal is transmitted and used at the decoder as the input of the LP synthesis filter in order to obtain the synthesized speech.
An innovative codebook in the CELP context, is an indexed set of N-sample-long sequences which will be referred to as N-dimensional codevectors. Each codebook sequence is indexed by an integer k ranging from 1 to M where M represents the size of the codebook often expressed as a number of bits b, where M=2b.
To synthesize speech according to the CELP technique, each block of N samples is synthesized by filtering an appropriate codevector from a codebook through time varying filters modeling the spectral characteristics of the speech signal. At the encoder end, the synthesis output is computed for all, or a subset, of the codevectors from the codebook (codebook search). The retained codevector is the one producing the synthesis output closest to the original speech signal according to a perceptually weighted distortion measure. This perceptual weighting is performed using a so-called perceptual weighting filter, which is usually derived from the LP synthesis filter.
The CELP model has been very successful in encoding telephone band sound signals, and several CELP-based standards exist in a wide range of applications, especially in digital cellular applications. In the telephone band, the sound signal is band-limited to 200-3400 Hz and sampled at 8000 samples/sec. In wideband speech/audio applications, the sound signal is band-limited to 50-7000 Hz and sampled at 16000 samples/sec.
Some difficulties arise when applying the telephone-band optimized CELP model to wideband signals, and additional features need to be added to the model in order to obtain high quality wideband signals.
Enhancing the periodicity of the excitation signal improves the quality in case of voiced segments. This was done in the past by filtering the innovative codevector from the fixed codebook through a filter having a transfer function of the form 1/(1xe2x88x92xcex5bzxe2x88x92T) where xcex5 is a factor below 0.5 which controls the amount of introduced periodicity. This approach is less efficient in case of wideband signals since it introduces the periodicity over the entire spectrum.
More specifically, in accordance with the present invention, there is provided a method for enhancing periodicity of an excitation signal produced in relation to a pitch codevector and an innovative codevector for supplying a signal synthesis filter in view synthesizing a wideband signal. In this periodicity enhancing method, a periodicity factor related to the wideband signal is calculated. Then, the innovative codevector is filtered in relation to the periodicity factor to thereby reduce energy of a low frequency portion of the innovative codevector and enhance periodicity of a low frequency portion of the excitation signal.
The device of the invention, for enhancing periodicity of an excitation signal produced in relation to adaptive and innovative codevectors for supplying a signal synthesis filter in view of synthesizing a wideband signal, comprises:
a) a factor generator for calculating a periodicity factor related to said wideband signal; and
b) an innovative filter for filtering the innovative codevector in relation to the periodicity factor to thereby reduce energy of a low frequency portion of the innovative codevector and enhance periodicity of a low frequency portion of the excitation signal.
According to a first preferred embodiment:
the innovative codevector is filtered with a transfer function of the form:
F(z)=xe2x88x92xcex1z+1xe2x88x92xcex1zxe2x88x921
where xcex1 is the periodicity factor derived from a level of periodicity of the excitation signal; and
the periodicity factor xcex1 is calculated using the relation:
xcex1=qRp bounded by xcex1 less than q
where q is an enhancement factor set for example to 0.25, and where       R    p    =                              b          2                ⁢                  v          T          t                ⁢                  v          T                                      u          t                ⁢        u              =                            b          2                ⁢                              ∑                          n              =              0                                      N              -              1                                ⁢                      xe2x80x83                    ⁢                                    v              T              2                        ⁡                          (              n              )                                                            ∑                      n            =            0                                N            -            1                          ⁢                  xe2x80x83                ⁢                              u            2                    ⁡                      (            n            )                              
where vT is the pitch codevector, b is a pitch gain, N is a subframe length, and u is the excitation signal, or
the relation:
xcex1=0.125 (1+rv), where
rv=(Evxe2x88x92Ec)/(Ev+Ec)
where Ev is the energy of the pitch codevector and Ec is the energy of the innovative codevector.
According to a second preferred embodiment:
the the innovative codevector is filtered with a transfer function of the form:
F(z)=1xe2x88x92"sgr"zxe2x88x921
where "sgr" is a periodicity factor derived from a level of periodicity of the excitation signal; and
the periodicity factor "sgr" is calculated using the relation:
xe2x80x83"sgr"=2qRp bounded by "sgr" less than 2q
where q is an enhancement factor set for example to 0.25, and where       R    p    =                              b          2                ⁢                  v          T          t                ⁢                  v          T                                      u          t                ⁢        u              =                            b          2                ⁢                              ∑                          n              =              0                                      N              -              1                                ⁢                      xe2x80x83                    ⁢                                    v              T              2                        ⁡                          (              n              )                                                            ∑                      n            =            0                                N            -            1                          ⁢                  xe2x80x83                ⁢                              u            2                    ⁡                      (            n            )                              
where vT is the pitch codevector, b is a pitch gain, N is a subframe length, and u is the excitation signal, or
the relation:
"sgr"=0.25 (1+rv), where
rv=(Evxe2x88x92Ec)/(Ev+Ec)
where Ev is the energy of the pitch codevector and Ec is the energy of the innovative codevector.
The present invention further relates to a decoder for producing a synthesized wideband signal, comprising:
a) a signal fragmenting device for receiving an encoded wideband signal and extracting from this encoded wideband signal at least pitch codebook parameters, innovative codebook parameters, and synthesis filter coefficients;
b) an pitch codebook responsive to the pitch codebook parameters for producing a pitch codevector;
c) an innovative codebook responsive to innovative codebook parameters for producing an innovative codevector;
d) a periodicity enhancing device as described above, comprising the factor generator for calculating a periodicity factor related to the wideband signal; and the innovation filter for filtering the innovative codevector in relation to the periodicity factor;
e) a combiner circuit for combining the pitch codevector and the innovative codevector filtered by the innovation filter to thereby produce a periodicity-enhanced excitation signal; and
f) a signal synthesis filter for filtering that periodicity-enhanced excitation signal in relation to the synthesis filter coefficients to thereby produce the synthesized wideband signal.
According to the present invention, in a decoder for producing a synthesized wideband signal, comprising: a signal fragmenting device for receiving an encoded wideband signal and extracting from this encoded wideband signal at least pitch codebook parameters, innovative codebook parameters, and synthesis filter coefficients; an pitch codebook responsive to the pitch codebook parameters for producing a pitch codevector; an innovative codebook responsive to innovative codebook parameters for producing an innovative codevector; a combiner circuit for combining the pitch codevector and the innovative codevector to thereby produce an excitation signal; and a signal synthesis filter for filtering that excitation signal in relation to the synthesis filter coefficients to thereby produce the synthesized wideband signal; the improvement therein comprising a periodicity enhancing device as described above, comprising the factor generator for calculating a periodicity factor related to the wideband signal; and the innovation filter for filtering the innovative codevector in relation to the periodicity factor before supplying this innovative codevector to the combiner circuit.
The present invention still further relates to a cellular communication system, a cellular mobile transmitter/receiver unit, a cellular network element, and a bidirectional wireless communication sub-system comprising the above described decoder.
The objects, advantages and other features of the present invention will become more apparent upon reading of the following non restrictive description of a preferred embodiment thereof, given by way of example only with reference to the accompanying drawings.